The place where the Lord Jesus ascended to heaven was on the Mount Olivet, or of Olives, near Jerusalem (verse 12) and Bethany (Luke 24:50). Luke clarifies for the benefit of Theophilus and ourselves, that the Mount of Olives is "a Sabbath day's journey" from Jerusalem, which is one kilometre (the greatest distance a Jew might travel on the Sabbath in NT times).
His disciples returned to Jerusalem, and the eleven apostles went up to the upper room where they were staying, to await the arrival of the Holy Spirit as the Lord had commanded. This is the fourth and last time that the apostles are listed by name in the Bible (Matthew 10: 2-4, Mark 3: 16-19, Luke 6: 14-16), now missing Judas Iscariot.
During this time they met "in harmony" to pray as the Lord had taught them: "if two of you agree on earth concerning anything that they ask, it will be done for them by My Father in heaven" (Matthew 18:19).
Women gathered to pray with the apostles, among them being Mary, “the mother of Jesus”, this being the last time she is at all mentioned in the Bible. Here she is qualified as "mother of Jesus". It was through her that the Lord came into the world, acquiring human form and taking the name of "Jesus" in His humanity, and she was His mother only for this purpose.
Mary is never called "mother of God", which would be absurd, even blasphemous, for God (Father, Son and Holy Spirit) is an eternal being, the Son being the Creator of all things, visible and invisible (Colossians 1:16). Mary was a descendent of King David and may have had a very good character, we know little about it, but she was a sinner and needed a Saviour, like all of us (Luke 1:47). The disciples did not address their prayers to her, but she followed them in their prayers to God the Father in the name of the Lord Jesus.
The brothers of Jesus were also present. Coming after the reference to Mary makes it likely that these were actual sons of Mary and half-brothers of Jesus. Before His death and resurrection they had not believed in Him (John 7:5), but He appeared to at least one of them after His resurrection, James (1 Corinthians 15:7) who became a leader of the pioneer church in Jerusalem (Acts 12:17, 15:13, 21:18, Galatians 1: 9), and wrote the letter with his name which we have in the Bible..
In these days of waiting, many of the disciples were gathered with the apostles, totalling about one hundred and twenty people.
Peter then saw fit to address a matter that concerned him: as the "twelve" (apostles) was reduced to eleven, he suggested that a replacement needed to be chosen among them to make them twelve again. Notice that:
The Lord had chosen His apostles personally, and as far as we know, had not instructed Peter or any other to choose who should fill the "vacancy" left by Judas.
The passage in Psalm 109:6-20 is a generic curse against the accusers and detractors of King David, applying in principle to all who slander the righteous. It does not seem to be specific to Judas because his crime was not defamation, but "he served as a guide to those who arrested Jesus." Even if it were applicable, there does not appear to have been any urgency to make it yet.
Peter seems to have anticipated the direction of the Holy Spirit that was to come, by choosing a replacement at this time.
Peter, nevertheless, stood in the middle of that congregation to make his proposal.
Current translations treat verses 18 and 19 as a parenthesis written by Luke and not part of Peter's message, and contain further details about the suicide of Judas, not found in the Gospels. They seem to be at variance with the account given by Matthew, who wrote that Judas threw the money paid for his betrayal inside the temple and went out and hanged himself (Matthew 27:5). The chief priests then bought a field with the money, which came to be called "Field of Blood". So the prophecy in Jeremiah was fulfilled (Jeremiah 18:18, 32:6 and Zechariah 11:13).
Here in Acts we read that Judas used the money to buy a field, there was an accident and he died. How to reconcile these statements? The most plausible explanation is that Judas hanged himself with something he had on hand, probably his belt, on the branch of a tree. The branch or the belt broke and he fell to his death.
As to the money, the chief priests could not stand to gain money illegally. Jewish law demanded it to be returned to the donor, and if he insisted on not receiving it back, he was induced to spend it for the public good. The money still legally belonged to Judas and the priests in his absence used it to buy the potter's field to bury strangers as if Judas himself had purchased it.
Peter continued, saying it was necessary to choose a replacement for Judas Iscariot, to witness the resurrection of the Lord Jesus with the eleven apostles (Matthew 28:16-20). He declared that only who had been with them throughout the time that the Lord Jesus lived among them, from the baptism of John until His ascension, could qualify.
With this condition, Peter considerably reduced the number of candidates, and as a result, only two met it and so the choice was now between those two.
We only read that they prayed after this was done, and directed their prayer to "You, O Lord, who know the hearts of all" though they had been taught to pray to the Father in the name of the Lord Jesus. They asked the Lord to reveal to them which of the selected two He had chosen to fill the vacancy left by Judas in the ministry and apostleship of the twelve. Note that the Lord had personally chosen the twelve apostles from all of His first disciples, who were not many, and now they asked Him to choose one of two candidates they selected, who also probably were among the first disciples but had not originally been chosen.
Then they cast their lots, so the Lord might have a chance to influence the outcome between those two. We are not told whether throughout this process there was the presence and guidance of the Holy Spirit, nor the authority of the Lord Jesus. We find later that the person the Lord Jesus really chose as His apostle was neither of those two, but Paul, to whom He appeared later and was not one of them at all (Acts 9:15, 1 Corinthians 15:8-10).
Matthias was the winner of the draw, so “he was numbered with the eleven apostles”, making up “the twelve” by which the group was known (see John 20:14, and 1 Corinthians 15:5, where the group of eleven were called “the twelve” though Judas was not among them). This group called “twelve” is only mentioned once more, in Acts 6:2, but Matthias appears no more. In Revelation 21:14 we read that the wall of the city of God seen by John in a vision “had twelve foundations, and on them were the names of the twelve apostles of the Lamb”. We read in Galatians 1:1 “Paul, an apostle (not from men nor through man, but through Jesus Christ and God the Father who raised Him from the dead)…” so Paul was undoubtedly an apostle, which leaves Matthias out.
It is remarkable that the first chapter of Acts "bonds together" the four Gospels: Matthew ends with the resurrection of the Lord Jesus, Mark with His ascension, Luke with the promise of the Holy Spirit, and John with the promise of His second coming: we find all four events mentioned. Acts is the bridge between the Gospels and the Epistles.
12 Then they returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a Sabbath day's journey.
13 And when they had entered, they went up into the upper room where they were staying: Peter, James, John, and Andrew; Philip and Thomas; Bartholomew and Matthew; James the son of Alphaeus and Simon the Zealot; and Judas the son of James.
14 These all continued with one accord in prayer and supplication, with the women and Mary the mother of Jesus, and with His brothers.
15 And in those days Peter stood up in the midst of the disciples (altogether the number of names was about a hundred and twenty), and said,
16 "Men and brethren, this Scripture had to be fulfilled, which the Holy Spirit spoke before by the mouth of David concerning Judas, who became a guide to those who arrested Jesus;
17 for he was numbered with us and obtained a part in this ministry."
18 (Now this man purchased a field with the wages of iniquity; and falling headlong, he burst open in the middle and all his entrails gushed out.
19 And it became known to all those dwelling in Jerusalem; so that field is called in their own language, Akel Dama, that is, Field of Blood.)
20 "For it is written in the Book of Psalms: 'LET HIS DWELLING PLACE BE DESOLATE, AND LET NO ONE LIVE IN IT'; and, 'LET ANOTHER TAKE HIS OFFICE.'
21 "Therefore, of these men who have accompanied us all the time that the Lord Jesus went in and out among us,
22 beginning from the baptism of John to that day when He was taken up from us, one of these must become a witness with us of His resurrection."
23 And they proposed two: Joseph called Barsabas, who was surnamed Justus, and Matthias.
24 And they prayed and said, "You, O Lord, who know the hearts of all, show which of these two You have chosen
Act 1:25 to take part in this ministry and apostleship from which Judas by transgression fell, that he might go to his own place."
26 And they cast their lots, and the lot fell on Matthias. And he was numbered with the eleven apostles.
Acts chapter 1, verses 12 to 26